Tacheometers, the generic term for theodolite/distance measuring instrument combinations, are powerful surveying tools capable of measuring the angles and the distances between points. Typically, this is done by leveling the instrument, optically aligning a gimbaled telescopic sight upon a target, measuring the shaft angles of the sight once aligned, and measuring the distance between the instrument and the target. Shaft angles can be measured by graduated vernier techniques or by more advanced digital encoding techniques described in more detail in the section below entitled THEODOLITE. Techniques for referencing the instrument to level are also described therein. Techniques for measuring distances include parallax methods as well as phase shift techniques such as described in more detail in the section below entitled DISTANCE MODULE. The major drawback of the prior art in tacheometers has been that a small, lightweight, and highly accurate tacheometer has heretofore been unobtainable.
A preferred embodiment of the present invention has a photosensitive receiver positioned to receive external and internal light beams for producing first and second electrical signals respectively in response thereto. A limit detector is coupled to the receiver via a variable gain amplifier for generating first and second range signals in response to detecting the amplified first and second electrical signals attaining values outside of first and second ranges. The first range, corresponding to operable limits of the intensity of the external light beam, is larger than and includes the values of the second range. The second range corresponds to a desired dynamic range of the electrical signals. A balancing circuit, coupled to the receiver and positioned for varying the intensity of one of the light beams, causes the value of the second electrical signal to follow the average value of the first electrical signal. Since the value of second electrical signal follows the value of the first and has a smaller range, if the external light beam intensity varies gradually, a second range signal is generated. In response to a second range signal, the variable gain amplifier is coupled to a feedback loop for adjusting the amplified electrical signals to values within the limit detector ranges and distance measurements are continued. If the external light beam intensity attains a value outside of its operating limits, a first range signal is generated. In response to a first range signal, the variable gain amplifier is left uncoupled from the feedback loop, the balance circuit is inhibited and no distance measurement is made. The instrument is thus prepared to make distance measurements immediately upon the external light beam re-attaining a value within its operating limits.
In operation, the preferred embodiment of the present invention can make accurate distance measurements upon external light beam signals having gradually varying intensities. Further, the present invention identifies sudden losses of the external light beam and places the instrument in a hold mode of operation awaiting for the return of the external beam. The preferred embodiment is thus capable of making continuous and accurate distance measurements in a variety of adverse operating conditions.